As is well known in the art, cryogenic tank systems have an inner tank disposed within an outer tank for creating a space therebetween such that a vacuum can be maintained between the inner and outer tanks for insulation purposes. While the outer tank may be insulated in a variety of conventional fashions, the inner tank is wrapped with cryogenic insulation.
That cryogenic insulation is comprised of two elements. The first element is referred to in the art as a "paper" but, in fact, is made of a fibrous felt-like material. That fibrous felt-like material is referred to as a "paper", since the thickness thereof is similar to industrial papers and has in the past been made on paper-making machines.
The second element of the cryogenic insulation is a metal foil. These foils are quite thin and may be made of a variety of metals. However, the foil is often made of aluminum, since aluminum has a reflective surface and the foil is used as a radiation barrier in the cryogenic insulation.
While a number of methods are employed by the art in wrapping the paper and foil around the outer surface of the inner cryogenic tank in the more usual method, cryogenic insulation paper is unrolled from at least one roll thereof, and the paper is wrapped in serially disposed wraps around the inner tank. Likewise, cryogenic insulating metal foil is unrolled from at least one roll thereof, and the foil is serially wrapped onto the respective serial wraps of the paper. Multiple layers of such serial wraps are placed around the inner tank. In this regard, "serial wrap" means that a wrap around the outer surface of the inner cryogenic tank is placed substantially adjacent to a prior wrap and substantially adjacent to a succeeding wrap, as explained in more detail below.
In such wrapping, it is of substantial importance that the edges of a wrap of the metal foil do not contact edges of a preceding wrap of the metal foil or the edges of a succeeding wrap of the metal foil, and it is likewise important that the metal foil of a wrap in one layer does not contact the metal foil of a wrap in a preceding layer or a succeeding layer. If such contact occurs, heat may flow by conduction, either between metal foil wraps in the same layer or metal foil wraps in a preceding or succeeding layer. If heat can flow by conduction, then a "short" or "hot spot" occurs, and the insulation value at such "short" or "hot spot" is severely reduced. This is because the metal foil is intended to function as a radiation barrier in such cryogenic tank systems, and in order for that radiation barrier to be fully effective, such metal-to-metal contact of the metal foil must be avoided.
One of the most common methods in the prior art for wrapping the inner tank involves rotating the inner tank about its longitudinal axis while wrapping the tank with the paper unwound from a roll thereof and with metal foil unwound from a roll thereof. This is generally achieved by means of a pivot arm rotated about a central axis with a roll of the paper on each extremity of the pivot arm and a roll of the foil near each roll of paper. The pivot arm is rotated in a plane generally parallel to the longitudinal axis of the inner tank, while the inner tank is being rotated about that longitudinal axis.
By this arrangement, a wrap of paper is placed on the tank and a wrap of foil is placed on that wrap of paper to form one layer of wraps of a multi-layered insulation. Since there is a roll of paper and a roll of foil at each extremity of the pivoted arm, two combinations of foil and paper are simultaneously wrapped onto the rotating inner tank. The common prior art method may be referred to as the orbital method.
This prior art process, however, suffers from decided disadvantages. First of all, since each extremity of the pivot arm can carry only one paper roll and one foil roll, only two combinations of paper and foil, one at each extremity of the pivot arm, can be wrapped onto the outer surface of the inner tank during rotation of the inner tank. This results in a relatively slow and time-consuming wrapping procedure, which not only considerably slows production, but increases the cost of wrapping the cryogenic insulation around the inner tank.
Secondly, in order to avoid shorts or hot spots, as explained above, it is necessary to carefully control the wrap of the metal foil onto the wrap of the paper such that the foil of a wrap does not contact the foil of a preceding wrap or succeeding wrap or contact the foil of a preceding layer of wraps or a succeeding layer of wraps. This requires careful monitoring and control of the wrapping system.
While all of the above has long been known in the art, the art has not discovered methods for wrapping the inner tank in a manner which will both speed the wrapping of that inner tank and, at the same time, avoid the possibility of shorts or hot spots. It would, therefore, be of considerable advantage to the art to provide a method of wrapping cryogenic insulation around the inner tank where the speed of that wrapping can be considerably increased, e.g. at least doubled, and where the possibility of shorts and hot spots is essentially eliminated.